A previously tested prototype of a towing vehicle of the general type under consideration herein, which was reported on in the media has a hold-down mechanism or holder for the nose wheel defined by horizontal arms. Before the nose wheel is engaged these arms are positioned at a height which corresponds to the nose wheel diameter for the aircraft type to be towed and while in this position act as fixed contact surfaces for the top of the nose wheel. This prepositioning of the horizontal arms is controlled by a computer which stores the necessary positioning data for the nose wheel diameters of various aircraft types that are to be towed with the vehicle. The operator must punch aircraft type identifying information into the computer to enable the prepositioning of the horizontal arms. Actual field tests of this prototype have revealed that it has some deficiencies.
First the prepositioning of the horizontal arms hinges on whether the correct aircraft identifying information has been punched into the computer and is, therefore, subject to human error. If the hold-down arms are set at the wrong height they can collide with and cause substantial damage to either the nose wheel, or worse still, to components of the nose wheel undercarriage mounted immediately above the nose wheel, e.g. the steering cylinder, the nose wheel doors and the like. The danger of damage is especially high for certain types of aircraft on which these components are located only a short distance above the nose wheel, when the nose wheel suspension is fully compressed, or when the air cylinder of the nose wheel suspension is empty. Nevertheless, damage to the nose wheel undercarriage must be avoided at all cost.
It has also been observed that the prepositioned hold-down arms do not effect a sufficiently secure connection of the nose wheel to the towing vehicle. When the aircraft is being towed and especially during acceleration, the nose wheel can dynamically move relative to the arms, which is often magnified as a result of the tire's inherent resiliency. These relative wheel movements generate forces which can overstress the nose wheel undercarriage or work the nose wheel free from the nose wheel securing or clamping device, especially if the initial holding force was relatively small. The latter can occur because the nose wheel engaging members are not capable of following the nose wheel periphery into engagement by, in effect, resiliently applying a holding force to the wheel so that, for example, minor variations in the wheel diameter, say due to wear, do not affect the force with which the wheel is held.
More conventional aircraft towing vehicles, which utilize a tow bar attached to the nose wheel undercarriage, typically have a shear pin which breaks to separate the towing vehicle from the aircraft before the nose wheel undercarriage can be overstressed by excessive forces which might be generated during towing. Such towing vehicles have a relatively poor connection to the aircraft and they can jackknife, especially during braking. Hence such towing vehicles are suitable only for low speed towing of unloaded aircraft manned by the pilot or flight engineer. Because these towing vehicles operate at low speeds, an occasional disengagement from the aircraft is acceptable since the aircraft can be slowed and brought to a stop independently of the towing vehicle.
Towing vehicles of the kind, however, which raise the nose wheel off the ground and clamp it in position permit relatively high speed towing of even unmanned aircraft and make it feasible to efficiently tow fully loaded aircraft to and from the runway. For such towing vehicles, it is essential that the nose wheel remains firmly secured under all circumstances and that it not be allowed to work itself free, or to "climb out" of the clamping mechanism.
A further drawback of the earlier version of wheel clamping towing vehicles has been their reliance on electronic signals and data processing to control and guide the components of the lifting and securing mechanism, e.g. setting the height of hold-down arms by means of stored data for different aircraft types, which is preselected by the vehicle operator, and their reliance on sensors that monitor and signal the relative positions of the components. The surroundings of large commercial airports employing aircraft towing vehicles of this type are full of significant electronic background noise and/or interfering signals which may disrupt the proper functioning of the computer, sensors and other electronics needed on board such towing vehicles. This can lead to a malfunction of the lifting and securing mechanism such as the wrong prepositioning of the horizontal arms.
A yet further consideration for towing vehicles of this type is that the lifting mechanism must be capable of securing nose wheels having widely varying diameters. For practical purposes, the range of applicable nose wheel diameters encompasses the full range found on conventional aircraft used by the larger commercial and charter airline companies, e.g. from 31" (ca. 78 cm) at the low end (for a Boeing 757) to 49" (ca. 125 cm) at the high end of the range (for a Boeing 747). Other common commercial aircraft such as the Boeing B767, the Airbus A300, A310 and the Douglas DC 8 and DC 10 to name a few, have nose wheel diameters falling within this range.
A wheel gripping towing vehicle of this general type disclosed in German patent publication DE-OS 35 34 045 has a lifting and securing mechanism with a hold-down apparatus pivotably mounted on levers located above the lifter. The hold-down arms engage the nose wheel as it is engaged by the lifter and the former moves in tandem with the wheel. When the engaging or receiving operation is completed, the hold-down arms are hydraulically locked in their upper position in which they are located above i.e., higher than the apex of the nose wheel, particularly when used with nose wheels of a smaller diameter. Thus, hold-down arms are in danger of colliding with aircraft undercarriage components located immediately above the nose wheel. Further, the initial contact between the hold-down arms and the nose wheel occurs when the receiving operation is almost completed and they are then hydraulically locked in place. These hold-down arms too form a fixed abutment surface for the wheel in its secured position without actively generating a clamping force. Thus, the above discussed drawbacks are equally applicable to this towing vehicle.